A comb generator is a signal generator that produces a signal (referred to here as a comb signal) whose output spectrum consists of multiple harmonics of a fundamental frequency. The appearance of the output signal on a spectrum analyzer screen, resembling teeth of a comb, gives the device its name. In some cases, synchronous tones spanning a wide frequency bandwidth can be produced by a comb generator.
FIG. 1 illustrates an example frequency spectrum 100 of a comb signal including a plurality of tones or comb teeth 110 which are harmonically related to each other. The amplitudes of the various comb teeth 110 may be the same as each other, or different from each other as in the case illustrated in FIG. 1. Often, as shown in FIG. 1, the amplitudes of the comb teeth 110 gradually decline from the lower frequency comb teeth 110 to the higher frequency comb teeth 110.
Comb generators and comb signals find wide range of uses in RF and microwave technology. One common use is in broadband frequency synthesizers, where the high frequency comb teeth act as stable references and can be used directly, or to synchronize phase-locked loop oscillators. Comb generators and comb signals also may be used in testing of a device or system, for example to generate a complete set of substitute channels each of which carries the same baseband signal for testing a multi-channel communication system.
In some cases, a comb generator may produce a comb signal which can be used as a phase calibration standard for subsequent measurements of another unknown signal. In such cases, the comb generator may be referred to as a Harmonic Phase Reference (HPR) standard, which may be a repetitive pulse generator with an adjustable repetition rate, also called the fundamental frequency. In some cases, an HPR standard may be based on a nonlinear monolithic microwave integrated circuit (MMIC) or a step recovery diode in combination with a nonlinear transmission line. In that case, a comb signal is generated at the output of the HPR standard which has energy at every integer multiple of the input signal frequency. In general, the relative phases between the various comb teeth (e.g., comb teeth 110) may have any arbitrary values, but the phase relationship is time invariant.
In many situations, it is desired to be able to provide an accurate and complete measurement of the phase relationships between at some of the comb teeth 110 of a comb signal generated by an HPR standard using a receiver or measurement device. In some cases, it is desired to measure the phase relationships between a selected plurality of comb teeth 110 of a comb signal using a measurement device whose bandwidth is less than the total bandwidth spanned by the selected plurality of comb teeth 110.
It would also be desirable to provide a convenient and reliable method and system to measure and characterize a periodically modulated signal, and an output signal of a device under test (DUT) produced in response to the periodically modulated signal, using an HPR standard and a receiver whose bandwidth is less than the bandwidth of the periodically modulated signal itself and/or the bandwidth of the output signal. It would further be desirable to provide such a system and method which can provide accurate measurements of phase sensitive characteristics, such as the error-vector-magnitude (EVM), for a DUT.